1. Field of the Invention
The present invention relates to a vacuum servo unit for vehicle brake systems.
2. Related Art
A conventional vacuum servo unit or booster is disclosed in, for example, Japanese patent Laid-open Print No. Sho. 57 (1982)-130845 published on Aug. 13, 1982, without examination. In this vacuum servo unit, an input rod which serves for transmitting a force from a brake pedal to a power piston is divided into two axial portions which are telescopically connected. The purpose of such a division of the input rod is to advance the power piston for activating the vacuum servo unit automatically even when the brake pedal is not depressed.
In order to establish smooth relative movement between the two axial portions of the input rod, a sufficient amount of a lubrication material is provided between the rod portions.
However, because this telescopic connection is made outside a casing or housing in which the power piston is located, the lubrication material may sometimes drop down near the driver""s feet. Thus, the driver""s feet and the neighboring vehicle body floor may become dirty.
In light of the foregoing, a vacuum servo unit is desired which is free from the foregoing drawbacks.
In order to attain the foregoing objects, an exemplary embodiment of the present invention provides a vacuum servo unit for vehicle brake systems comprising:
a housing defining therein at least a pressure space;
a movable wall provided in the housing so as to be capable of advance and retract along an axial direction of the housing, the movable wall dividing the pressure space into a front chamber and a rear chamber;
a power piston coupled to the movable wall;
an input member fitted in the power piston so as to be movable back and forth along the axial direction of the housing;
an input rod having a front portion and a rear portion, the front portion being coupled to the input member, the rear portion being coupled to a brake operation member, the front portion being in engagement with the rear portion within the power piston in such a manner that the front portion is movable relative to the rear portion, the input rod being movable along the axial direction of the housing together with the brake operation member;
a valve mechanism having an atmosphere valve and a negative pressure valve, the valve mechanism establishing a fluid communication between the rear chamber and the atmospheric pressure by opening the atmosphere valve which provides an advancing power to the power piston in simultaneous with an interruption a fluid communication between the rear chamber and the front chamber which is caused by closing the negative pressure valve when a stroke of the input member causes by the depression of the brake operation member exceeds a set value;
an output member establishing an outer-housing outputting of the advancing power of the power piston caused by the depression of the brake operation member; and
an actuator providing a different advancing power to the power piston independent of the brake operation member.